One type of conventional raster out scanner (ROS) system writes data or graphics by a single laser beam which is scanned by a rotating polygon across each scan line position of a charged photosensitive surface such that discrete scan line positions of the drum are discharged. Higher resolution, higher performance laser ROS systems will require much higher data rates than conventional systems and much higher polygon rotation speeds, assuming existing single channel ROS architectures. A multi-channel scanner architecture wherein several scan lines are scanned at once will significantly reduce both the high data rate per channel and high scan rate speed problems since the scan speed of each scan spot can be slowed down whereby each data spot or pixel of a scan line can dwell on the photosensitive medium surface for a longer time period compared to the equivalent single channel configuration. TIR modulators are well suited for multi-channel output scanners and have been proposed for such systems.
A drawback of multi-channel scanners is that errors in photoreceptor speed will have a longer time to build up corresponding errors in placement of the next group of scan lines with respect to the last group of scan lines. There is, therefore, need for a multi-channel scanner that will rapidly shift the position in the slow scan direction of each group of scan lines relative to the preceeding group of scan lines in response to detected changes or errors in photosensitive surface position resulting from undesired photosensitive surface velocity variations.